In order to reduce CO2 emission from the viewpoint of global environment conservation, improving the fuel efficiency of an automobile by reducing the weight of the automobile body while maintaining the strength of the whole automobile body has continuously been an important task in the automobile world.
In order to reduce the weight of an automobile body while maintaining the strength of the whole automobile body, it is effective to reduce the thickness of a steel sheet by increasing the strength of the steel sheet which is used as a material for automobile parts. Here, many of automobile parts are formed by using, for example, a press forming method or a burring method. Therefore, a high-strength steel sheet which is used as a material for automobile parts is required to have not only a desired strength but also excellent formability.
Nowadays, there is a growing trend toward using a high-strength steel sheet of over 1180 MPa grade as a material for the skeleton of an automobile body. Since it is difficult to ordinarily perform conventional press forming on a high-strength steel sheet of over 1180 MPa grade due to its high degree of difficulty of forming, a processing method mainly including bending such as roll forming is used when such a steel sheet is formed. Therefore, bending formability is one of the important properties when steel of over 1180 MPa grade is used.
Also, examples of important properties required for a material for automobile parts include anti-crash property. At the time of an automotive collision, a strain rate applied to automobile parts composed of steel sheets reaches about 103/s. Therefore, automotive parts such as a pillar, a member, and a bumper are required to have sufficient anti-crash property for achieving occupant safety when a collision occurs by some chance while an automobile is traveling. That is, it is necessary to achieve the collision safety of an automobile by manufacturing automobile parts described above by using a high-strength steel sheet having sufficient anti-crash property for excellently absorbing collision energy even in the case where the high strain rate as described above is applied at the time of a collision.
In response to such requirements, for example, Patent Literature 1 discloses a technique regarding a steel sheet of 1180 MPa grade excellent in terms of formability and anti-crash property. Although it is possible to improve formability such as elongation and stretch flange formability according to the technique described in Patent Literature 1, bending formability and spot weldability, which is one of the important properties required for an automotive steel sheet, are not discussed. In addition, in the case of the technique disclosed in Patent Literature 1, since tempering equipment is needed, there is also a problem regarding equipment costs.
Patent Literature 2 discloses a technique regarding a high-strength steel sheet having good bending formability, good spot weldability, and so forth. However, in Patent Literature 2, since anti-crash property is not evaluated, the high-strength steel sheet may not have sufficient anti-crash property. In addition, in Patent Literature 2, there is almost no disclosure regarding a steel sheet with over 1180 MPa tensile strength.
PTL 1: Japanese Unexamined Patent Application Publication No. 2012-031462
PTL 2: Japanese Unexamined Patent Application Publication No. 2011-132602